SHORT-TERM NITRATE EFFECTS ON HYDROPONICALLY-GROWN SOYBEAN CV BRAGG AND ITS SUPERNODULATING MUTANT .2. DISTRIBUTION AND RESPIRATION OF RECENTLY-FIXED 13C-LABELED PHOTOSYNTHATE

Fully symbiotic or nitrate treated (3 d, 4.0 mol m-3) soybean (Glycine max [L.] Merr.) cv. Bragg and a nitrate tolerant supernodulating soybean mutant nts 1007 were exposed to C-13 enriched CO2 for a period of 10 h. During this period and for the subsequent 24 h, continuous measurements of (CO2)-C-13 and (CO2)-C-12 evolution of their root systems were undertaken. Three harvests during the experiment allowed determinations of the distribution of recently fixed carbon in different plant organs. These measurements indicated higher dependence of N2 fixation in nts 1007 on recently fixed carbon (RFC) by showing elevated RFC concentrations in nodules as well as their augmented respiration. Root respiration of both genotypes was generally more reliant on stored carbon. Nitrate induced in all measured parameters a clear response in the mutant analogous to the wild type. but quantitative differences remained throughout. Nodule respiratory activity, the relative specific activity (RSA), and the utilization of RFC were substantially reduced, but remained higher in nts 1007 than in Bragg, while the demand of roots for RFC increased in Bragg more than in the supernodulator. The elevated carbon requirement of the nodule complement of the mutant and a high dependence on recently fixed carbon could be attributed to higher nodule growth and maintenance costs of the supernodulating genotype and were not associated with augmented nitrogen fixation activity. This less efficient utilization of carbon and the associated almost parasitic character of the nodule complement of nts 1007 is considered to be the cause of reduced growth of the mutant. No evidence was found for a physiologically based nitrate tolerance in terms of nitrogen fixation.